a. Field of the Invention
The invention relates to a SQUID magnetometer made by using a thin-film technology and particularly useful for a device for measuring weak magnetic fields, wherein the magnetometer is of the kind having a d-c SQUID with a SQUID loop and a super conducting flux transformer for the inductive coupling of a measuring signal into the SQUID.
b. Description of the Prior Art
A measuring device with a SQUID can be found in the publication "IEEE Transactions on Magnetics," Vol., MAG-17, No. 1, January 1981, pages 400 to 403.
Superconducting quantum interferometers which are generally known in the art as SQUID's (Superconducting QUantum Interference Devices), are used for the measurement of very weak magnetic fields as described in "J. Phys. E.: Sci. Instrum.," Vol. 13, 1980, pages 801 to 813; and "IEEE Transactions on Electron Devices," Vol. ED-27, No. 10, October 1980, pages 1896 to 1908. These interferometers are particularly preferred in the field of medical technology, and in particular, magnetocardiology and magnetoencephalography, since the field intensities produced by magnetic heart and brain waves are in the order of about 50 pT and 0.1 pT, respectively. (See e.g. "Biomagnetism - Proceedings of the Third International Workshop on Biomagnetism, Berlin 1980," Berlin/New York 1981, pages 3 to 31).
For measuring such biomagnetic fields, measuring devices are known which can be designed with one or more channels (see, for instance, DE-OS No. 32 47 543). Depending on the number of channels, these devices contain at least one SQUID magnetometer.
Such a magnetometer can be made with thin-film technology, as described in the "IEEE Trans. Magn." reference mentioned above. It has a relatively wide ring-shaped SQUID loop of superconducting material which forms a quasi-square or rectangular frame about a corresponding shaped central coupling hole. On one side, this loop is interrupted by a narrow transversal slot which leads to the outside and is almost completely overlapped by a strip-shaped conductor run. In the free region of the slot (i.e., the region not covered by the conductor), the SQUID loop is closed with two Josephson tunnel elements characteristic for a d-c SQUID. The magnetometer also includes a frame-shaped coupling coil formed of superconductive turns surrounding the coupling hole. In this-known embodiment, the SQUID loop also serves as the supporting base plane for the coupling coil. This coupling coil, together with at least one superconducting gradiometer coil connected thereto forms a flux transformer, by which a measuring signal to be detected can be coupled into the SQUID via the SQUID loop. The coupling losses are here proportional to the self-inductance of the strip line which is formed by the coupling coil and the SQUID loop. The self-inductance is given by the following relationship: EQU L=u.sub.o .multidot.1.multidot.d.sub.iso /WK
where 1 is the length of the coupling coil, d.sub.iso the distance between the SQUID loop and the coupling coil, W the track width of the coupling coil and K the so-called fringe factor which depends on d.sub.iso /W. The self-inductance L is therefore a function of 1 and d.sub.iso /W. More particularly, L is proportional to d.sub.iso /W.
It has now been found that such magnetometers, especially for multichannel measuring devices, can be realized with satisfactory properties only with great difficulty. For example, the dimensions of the SQUID loop, for one, must be chosen at least large enough so that the turns of the relatively extensive coupling coil can be put on this loop. However, wide loop strips effect the properties of the SQUID adversely. Thus, undesirable resonances are observed between the straight conductor sections of the turns of the coupling coil and the SQUIDS located below. In addition, the parasitic inductance at the slot of the SQUID loop is relatively large. Because of this parasitic inductance, the coupling of the magnetic flux from the coupling coil into the SQUID is impeded correspondingly.